1. Field of the Invention
The present invention relates to apparatus for remotely sensing variables substantially-simultaneously at a plurality of locations. More particularly, this invention pertains to a fiber optic sensor array architecture therefor.
2. Background of the Prior Art
Current methods for the remote sensing of variables such as acoustic pressure at multiple locations employ electronic means (piezoelectric transducers or "hydrophones", low-noise electronic preamplifiers and either analog or digital multiplexing). Such hardware is generally located within a waterproof, oil-filled hose that may be towed behind a ship.
Sensor arrays of the above-described type require close proximity between the preamplifier and the transducer. The preamplifiers, low-noise devices with high-input impedances, are prone to pick up extraneous signals. Furthermore, it is the present trend to utilize smaller diameter hoses which, of course, only exacerbates the pick-up problem. Conventional array designs locate the bulk of the electronic hardware within the system's "wet" end and therefore drive up the wet end cost. This is quite undesirable since the wet end of the system is subject to extreme mechanical and environmental stresses and, in fact, is often subject to damage or loss. Coaxial cable or twisted wire pairs, commonly employed in telemetry schemes, are subject to crosstalk when multiple lines are required (that is, for arrays containing large numbers of sensors) and especially when the lines must be packaged in small-diameter hoses.
Attempts have been made to perform substantially the same functions by means of fiber optic sensors and telemetry. Three such approaches have been investigated including (1) time-division multiplexed, (2) frequency-division multiplexed and (3) Fabry-Perot (recursive) designs. Each of such approaches is subject to significant shortcomings. Fabry-Perot sensor arrays (which employ partially-reflective splice joints) are subject to unacceptably high levels of crosstalk in certain significant applications. Time-division multiplexed recursive array architectures that employ lightly-tapped directional couplers also encounter crosstalk problems. Non-recursive sensor array architectures (either time or frequency division multiplexed) can achieve acceptable crosstalk levels but lack optical power efficiency due to the losses that occur at dead-end paths on the return fiber bus.